1. Field of the Invention
This invention relates to the field of illuminated sign systems. In particular, the invention concerns the edge lighting of signs using light emitting diodes or LEDs. The invention utilizes high brightness LEDs aligned so as to inject light directly into the edge of a transparent plate or sheet. The result is a low power, low profile, high visibility, high durability illuminated sign system which is particularly useful for "exit" signs.
2. Prior Art
Light Emitting Diodes or LEDs have been previously applied to the field of illuminated sign systems. A common LED based sign system uses an X-Y matrix of closely spaced LEDs arranged so as to emit light in the direction of the observer. The LEDs in the matrix are selectively illuminated, forming letters, words or graphic symbols. When an LED matrix sign is viewed from distances of less than 100 feet, the graphic patterns produced by the LEDs appears as lines composed of discrete points or dots of light; even at distances in excess of 100 feet the human eye does not readily "merge" the discrete points of light into a continuous form. Although the sign utilizing an X-Y matrix are quite legible, the graphic appearance is not preferred by building owners, architects and specifiers, who recommend evenly illuminated graphics.
LEDs emit light at a wavelength which is predetermined by their semiconductor composition. An illuminated sign system based on a LED matrix is thus limited in the colors that can be displayed. However, there are at least a few color selections available. Although some LED matrix systems allow the user to modify or customize messages for display these systems do not represent an acceptable replacement for illuminated graphic panels, which can be decorated using a wide variety of colors and/or patterns.
Graphic panels for purposes of this disclosure include any panel surfaces having graphics applied by some means such as painting, silk screening, etching, etc., to define indicia by negative or positive means. Graphic panels can be illuminated by several methods of direct or indirect as well as edge lighting. Lighting systems generally direct light from a light source onto either the viewing surface of the graphic panel or through a translucent graphic panel disposed behind the graphic information to be viewed. Edge lighting systems, on the other hand, use a light source to inject light into the edge of a transparent or translucent graphic panel, whereby the panel is caused to glow and any graphics in contact with the panel are made apparent. For example, a message painted on the back side of a panel viewed from the front can be illuminated in this manner, or a panel can be caused to glow generally, the indicia being defined by or between opaque areas on the front side.
LEDs emit a very narrow beam of light, making uniform lighting of large panel surfaces difficult or impossible. LED illumination systems utilizing direct illumination or illumination from behind a translucent diffuser panel, i.e., where light emitted by the LEDs is aimed at a graphic panel in a direction perpendicular to the plane of the graphic panel, produce only marginal uniformity of light intensity over the entire graphic panel unless large numbers of LEDs are used. A diffuse panel helps to spread the LED beams, but diffusers attenuate the light from the source, and thus adversely affect efficiency. A diffuser adds to the cost of the illumination system, adds to the cost of operating the system, and adds dimensional thickness.
The present invention employs the durability and long service life of high brightness LEDs which are now available, and provides more uniform lighting of a graphic panel using an edge lighting technique. Edge lighting takes advantage of internal reflections within the graphic panel, allowing the entire panel to be evenly illuminated using fewer LEDs than conventional methods. Since fewer LEDs are used, the invention results in a cost savings during both manufacture and operation of the sign. The LEDs can be mounted in a row at a housing or mounting portion of a panel which protrudes from a wall or ceiling, resulting in a durable, efficient and aesthetically pleasing illuminated sign structure.
Edge illumination has been employed using fluorescent and incandescent light sources. Systems using fluorescent light sources have several disadvantages since they require complicated circuitry including a ballast reactor (a coil of wire wrapped around a heavy iron core). Fluorescent lighting systems require substantial space for the location of the fluorescent lamps and circuitry, increasing the profile of the sign systems. Additionally fluorescent systems are heavy and fragile when compared to the lighting apparatus of the invention. Incandescent illumination systems also have significant disadvantages since they are relatively inefficient and fragile, and produce a great deal of heat.